The field of the invention is communications networks, and more particularly, bit oriented communications networks for use in industrial environments.
Industrial installations typically contain a large number of electrical devices which must be connected together. These devices can generally be classified as input resources and output resources. Examples of input resources typically used are switches of many types, relay contacts, and signals generated by controllers used in the installation. Typical output resources include relay coils, indicator lamps, and again, signals fed into controllers. The nature of the connections between the input and output resources is largely bit oriented, consisting of single bits, or on/off values for the various switches, relays, and indicators.
Originally, connections were made between the various input and output resources using discrete wiring. Individual wires were run between each input point and output point. This of course results in large wire bundles and enormous amounts of wiring which is very difficult to change should future alteration be desired.
Communications networks have been used to perform Input and Output (I/O) from controllers, however this approach requires a large and usually quite expensive controller to, in effect, replace the discrete connectios. in operation, the controller must first read the values of the input resources into is own processor and then transmit to the output resources.
While this approach is generally satisfactory, there are several drawbacks. First of all is the cost of obtaining and programming the controller. Secondly, if there are many input and output resources, the processing loop of the controller can become very long, which means that servicing time needed to detect a change in the state of an input resource is equally long.
Other examples of known communications networks include many types of token passing logical ring networks, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) networks such as Ethernet, and Dynamic Time Slot Allocation (DTSA) networks. Prior applications of these communications networks have been primarily word oriented. In addition, most prior networks communicate on a point-to-point basis, with messages specifically addressed from one network node to another. This is also not practical for large bit level communications systems, as the large number of messages required would slow servicing time to unacceptable levels.